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High Temperature Acoustic Fluid Velocity Instrumentation Sensor Development
Keywords: high-temperature high-pressure , fluid velocity sensor, reliability and durability
Development of downhole sensors for high temperatures and pressure environment present numerous challenges. These challenges are presented in the area of material selection, processes used for manufacturing, sensor dimension restrictions, testing and verification of the sensors. A sensor development for subsurface formation fluid sample evaluation requiring a reliable operation up to 38,000 Psi and 450F was undertaken. This presentation focuses on the sensor development’s technical challenges and solution for measurement of acoustic velocities of the sampled formation fluid. During the course of development of the acoustic sensor for high temperature environment, numerous wire to crystal bonding methods were investigated which included soldering, sintering and epoxying. Temperature cycling test following the environmental stress specification (i.e. sensor temperature, shock and vibration mission profile) was performed to evaluate product lifetime reliability and durability as well as to catch dominant early-term defects by inducing failure through thermal fatigue. Evidence of partially lifted solder joint was observed on the solder connection of the shielded wire on the sensor body after the cycling test between two operating temperature extremes. Nondestructive failure analysis was performed to inspect components and evaluate materials. The signal strength loss and reduced capacitance were caused by the solder bridging observed in the body of the sound speed sensor. In this paper, possible delamination, discontinuities and mismatch differences in materials characteristics of materials due to coefficient of thermal expansion and contraction will be discussed. The partially lifted solder joint is also considered a critical reliability concern. For functional testing of the sensor, the acoustic signal was injected in a metallic block and the returned signal was captured and analyzed. The returned signal must have sufficient strength level in order to capture and measure sound speed through the target sample medium. Several returned acoustic signals measurements were captured as a function of temperature to investigate the attenuation calibration of the signal as a function of temperature. Besides temperature, the signal strength of the reflected sound wave was also affected by epoxies and potting compounds used in the construction of the sensor. Interestingly, survival at elevated temperatures for 100 hour-long does not insure survival upon cool down cycles because of the effects of differential coefficients of thermal expansion that can create cumulative cyclical stresses between the electrical connection and its adjacent structure, which can disconnect the wire attachment. In principle, one could use a solder that melts below the target temperature as long as it remains fully encapsulated in epoxy but, in our experience, cool down becomes an issue in this case. To reliably and predictably survive environmental shock and vibration forces propagated to the sensor, we strive to minimize mass to minimize forces and stresses coupled to the sensor structural components and material interfaces at very high acceleration levels. The overall results of various bonding construction methods and the corresponding sensor acoustic response over the broad rated pressure and temperature range will be presented along with final conclusions.
Imran Moton, Project Technical Lead
Baker Hughes, Inc.
Houston, TX
USA


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